1. Field of the Invention
The invention concerns radar surveillance antennas and, more particularly, in such antennas, a system for identifying targets by encoded interrogations, the antenna of this system being associated with the antenna of the surveillance radar.
2. Description of the Prior Art
Radars can be used to detect the presence of objects or targets and to determine certain of their characteristics such as their distance, altitude and speed. However, they cannot be used in wartime to determine whether the target is a friend or a foe. Such determining is done by using a system that "interrogates" the targets by sending them encoded signals which are detected by these targets. The targets may then emit encoded signals, indicating their respective category, to the interrogator system. A target that does not "respond" appropriately to the encoded signals is considered to be a foe.
An interrogator/responder system such as this, more commonly known as an I.F.F. (Identification Friend or Foe) system, is much used in peacetime for it enables a radar operator to easily identify the aircraft with which he is in radio contact by asking it to emit a determined encoded signal. This encoded signal appears in a particular form on the radar screen in the vicinity of the corresponding radar signal. For obvious reasons, the antenna of the I.F.F. system is borne by the radar antenna, and this results in a very bulky and heavy unit.
To overcome this problem, it has been proposed to use a single antenna for both the radar and the I.F.F. functions. An antenna such as this is, for example, made by means of a so-called primary source of radar signals which illuminates a reflector. Dipoles are associated with the primary source. These dipoles emit I.F.F. signals and also illuminate the radar reflector. Such an approach is not entirely satisfactory for the I.F.F. channel cannot be optimized while the level of the cross-polarized signals is too high to comply with certain technical standards laid down in aeronautics.